The Heterogeneity of Ly6Chi Monocytes Controls Their Differentiation into iNOS+ Macrophages or Monocyte-Derived Dendritic Cells

Inflammation triggers the differentiation of Ly6C(hi) monocytes into microbicidal macrophages or monocyte-derived dendritic cells (moDCs). Yet, it is unclear whether environmental inflammatory cues control the polarization of monocytes toward each of these fates or whether specialized monocyte progenitor subsets exist before inflammation. Here, we have shown that naive monocytes are phenotypically heterogeneous and contain an NR4A1- and Flt3L-independent, CCR2-dependent, Flt3(+)CD11c(-)MHCII(+)PU.1(hi) subset. This subset acted as a precursor for Fc gamma RIII+PD-L2(+)CD209a(+), GM-CSF-dependent moDCs but was distal from the DC lineage, as shown by fate-mapping experiments using Zbtb46. By contrast, Flt3(-)CD11c(-)MHCII(-)PU.1(lo) monocytes differentiated into Fc gamma RIII+PD-L2(-)CD209a(-)iNOS(+) macrophages upon microbial stimulation. Importantly, Sfpi1 haploinsufficiency genetically distinguished the precursor activities of monocytes toward moDCs or microbicidal macrophages. Indeed, Sfpi1(+/-) mice had reduced Flt3(+)CD11c(-)MHCII(+) monocytes and GM-CSF-dependent Fc gamma RIII+PD-L2(+)CD209a(+) moDCs but generated iNOS(+) macrophages more efficiently. Therefore, intercellular disparities of PU.1 expression within naive monocytes segregate progenitor activity for inflammatory iNOS(+) macrophages or moDCs.